With the increase of the size and the integration degree of semiconductor memory chips, demands are growing that plastic sealing molds for packaging chips be smaller and thinner. A semiconductor package having an LOC structure has been proposed in line with this. A lead called the "bus bar" is used in an LOC structure type semiconductor package to accommodate a number of wires in a limited space. The bus bar serves as an inner lead for supplying a voltage (e.g., a power supply voltage (Vcc) and a reference voltage (Vss)) to a semiconductor chip, and is disposed so as to traverse the chip. Since the bus bar can be connected to any point of the chip by short-distance wire bonding to supply a voltage, it can be effectively employed to reduce noise and increase processing speed.
However, since bonding wires for connecting inner leads are provided with bonding pads traversing the bus bar, these wires may fall or may be laterally pushed and deformed by a mechanical impact during the assembly process, the weight of such wires, or the pressure of an injected resin sealing agent. In this manner, the bonding wires may contact the bus bar, or may contact each other, thus causing a short-circuit.
If bonding wires are wired so as to form a large loop in order to avoid contact thereof with a bus bar, the wires become longer, thus increasing manufacturing costs. Furthermore, the looping of bonding wires makes it more difficult to reduce the size and thickness of semiconductor devices.
To solve the above problems, various techniques for preventing short-circuiting due to the contact of two bonding wires or of a bonding wire and a bus bar have been proposed in the past. FIGS. 4 and 5 show an example of those conventional techniques.
One such technique is disclosed in Published Unexamined Patent Application (PUPA) No. 4-114438. FIG. 4 is a perspective view of an LSI (large scale index) package which is partially cut away. FIG. 5 is a sectional view taken along the shorter axis of the package in FIG. 4. As shown in FIGS. 4 and 5, a semiconductor chip 2 is sealed in a package main body 1. A pair of insulating films 3 is bonded to the principal surface of the semiconductor chip 2 so as to extend along its longer sides. The insulating films 3 are thin films of, for instance, polyimide type resin, and are bonded to the principal surface of the semiconductor chip 2 by an epoxy or polyimide type adhesive. A plurality of bonding pads 5 are formed on the central portion of the principal surface of the semiconductor chip 2 between the pair of insulating films 3 so as to be arranged along the longer sides of the chip 2. A plurality of inner leads 4 are bonded to the insulating films 3 so as to be arranged along the longer sides of the semiconductor chip 2. Each of the inner leads 4 is electrically connected to the corresponding bonding pad 5 on the semiconductor chip 2 through a bonding wire 6.
Among the plurality of inner leads 4, the leads 4 located at both ends of each side are electrically connected to each other through a bus bar 7. The bus bar 7 has a bracket-like pattern which extends along the two shorter sides and the one longer side (on the center side ) of the insulating film 3, and is bonded to the insulating film 3 by, for instance, an epoxy or polyimide type adhesive. The bus bar 7 is made of a thin (several tens of micrometers) conductive foil of Cu (copper), etc., plated with a metal of Au (gold), etc. Since the bus bar 7 is quite a bit thinner than the inner lead 4, which has a thickness of 150-250 .mu.m, a sufficiently long distance is located between the bus bar 7 and the bonding wires 6 traversing it, as especially shown in FIG. 5.
PUPA Nos. 3-129840 and 5-13654 disclose a technique in which an insulating substance is applied or stuck on a bus bar or a part of a land on which a semiconductor is mounted, and bonding wires are glued and fixed to the insulating substance.
Technical Disclosure Bulletin, Vol. 34, No. 1, which was published in June 1991 by IBM (International Business Machines) Corporation, discloses a technique in which bonding wires are connected so as to traverse a bus bar, and a circular insulating film, whose one or two surfaces are sticky, are arranged on the bus bar under the bonding wires. When an insulating double-sided adhesive film is used, the bonding wires are stuck and fixed to the bus bar by means of the insulating film.
PUPA No. 3-204965 discloses a technique in which an insulating film made of an epoxy resin is adhered on the top surface of the inner leads (or bus bar) by thermocompression bonding so that metal wires do not come into direct contact with the bus bar even if wire bonding is performed so that the metal wires traverse the bus bar.
PUPA No. 3-255655 discloses a technique in which a lead portion (wire connection portion) located under a bonding wire that traverses it and connected to another lead is coated with a thermosetting resin so that short-circuiting does not happen even when a bonding wire deformed by, for instance, a mechanical impact during the assembly process is brought into contact with the above lead portion.
However, according to the conventional techniques, a bus bar needs to be much thinner uniformly than the inner leads, an insulating adhesive needs to be applied to the top surface of a bus bar, wires coated with an insulating substance need to be used, and/or the attachment of an insulating film is needed.
It is believed that a circuit device as defined herein, which would overcome the aforementioned disadvantages, would represent a significant advancement in the art.